Objectives & key points. Outline of muscle lectures

Transcription

1 Objectives & key points Identify contractile and regulatory proteins Describe excitation-contraction coupling Recognize the role and source of Ca in muscle contraction Identify energy sources for muscle contraction Describe mechanisms that regulate muscle contraction & relaxation Contrast structural, energetic & regulatory aspects of 3 muscle types 1 Outline of muscle lectures Structure Excitation-contraction coupling Energetics & mechanics of contraction 2 Review of structure Skeletal muscle cells (fibers) made of myofibrils Myofibrils: Unit of repeating pattern of thick & thin filaments Thick filaments Myosin Orientated opposite directions Middle of sarcomere Overlap with thin filaments Thin filaments Actin, tropomyosin, troponin 2 sets per sarcomere One end connected to Z line Interconnecting proteins One end overlaps portion of thick filaments A band 3 1

2 Review of structure Sarcomere: smallest functional unit of muscle Area between 2 adjacent Z lines Striated pattern dark A bands Bisected X H band light I bands Bisected X Z line 4 Skeletal muscle structure Contractile proteins Actin (2 helical chains) Globular protein Has binding site for myosin Myosin (6 subunits) 2 Heavy chains Elongated alpha-helical rod Head binds ATP & actin ATPase activity 4 Light chains Regulatory ATPase activity Motor function 5 Skeletal muscle structure Regulatory Proteins Tropomyosin 2 alpha helices coiled around Sit in groove of actin chains Cover myosin binding sites Troponin Heterotrimer Small globular proteins at intervals on tropomyosin T: binds Tropomyosin I: binds actin & Inhibits actomyosin ATPase C: binds Ca

3 Skeletal muscle structure Structural proteins Dystrophin-glycoprotein complex Links actin to extracellular matrix Structural support & strength to muscle fibril Duchene muscular dystrophy y Titin Anchors myosin to actin network Keeps neat striation pattern 7 Skeletal muscle structure Sarcotubular System: T tubules & sarcoplasmic reticulum In-foldings of plasma membrane Penetrate muscle fiber at A-I junction Lumen continuous with ECF Helps the spread of action potential Associated with cisternae Specialized region of SR Ca ++ stores 8 Excitation-contraction coupling skeletal How is muscle contraction initiated? Somatic Nervous System Motor neuron Spinal cord or brainstem Myelinated axons AP propagated at high velocity Innervate many muscle fibers Neuromuscular junction Each muscle fiber controlled by branch from 1 motor neuron MOTOR UNIT: minimum unit of contraction 1 MN 2-3 fibers (larynx) 1 MN 10 fibers (eye) 1 MN fibers (gastrocnemius) 9 3

4 Electrical characteristics Motor neuron releases Acetylcholine AP reaches axon terminal Plasma membrane depolarization Opening of voltage-sensitive Ca ++ channels Ca ++ influx to axon terminal Ach release into cleft Ach binds to cholinergic receptor on muscle fiber end plate Nicotinic (N1) receptor 10 Excitation-contraction Skeletal muscle Acetylcholine binding triggers skeletal muscle action potential Ach nicotinic receptor Ligand gated-na channel Na+ influx muscle end plate depolarization Depolarize adjacent plasma membrane Open voltage-gated Na+ & K+ channels Spread of AP by T tubules Acetylcholinesterase receptor-bound Ach End-plate Ion channels close Returns to resting potential Ready to respond to Ach again 11 Contractile responses T tubule spreads action potential leading to SR Ca ++ release AP conducted via T tubule Activates voltage sensors: Dihydropyridine receptors L-type voltage-gated Ca ++ channels Undergo conformational change Opens SR Ca ++ channel Ryanodine receptor cytosol [Ca ++ ] 100-fold 12 4

5 RYR1 SERCA Skeletal & Cardiac contraction cytosolic Ca++ Binds troponin C Troponin conformational change Troponin I to actin binding Displaces tropomyosin from groove Uncover myosin binding sites (actin) Myosin-actin interaction Cross-bridge formation 14 Cross bridge cycle 15 5

6 RYR1 SERCA Contractile responses Muscle relaxation X Cytosolic Ca ++ Ca removed Across cell membrane (minor) Na + /Ca ++ exchanger Reuptake into SR (major) Ca ++ /Mg ++ ATPase (active transport) Bound by calsequestrin & calreticulin Ca ++ - binding proteins in triad junction cytosolic Ca ++ Ca ++ removed from troponin Restore tropomyosin blocking action Cover myosin-binding site on actin Actin/myosin interaction stops 17 Excitation-contraction coupling: Clinical Perspective Botox Curare 18 6

7 Energy & metabolism Muscle contraction & relaxation require energy (ATP) Cross-bridge cycling Power stroke & myosin dissociation from actin Relaxation Removal of cytosolic Ca ++ Ca ++ /Mg ++ ATPase pumps Ca ++ into SR No ATP Thick & thin filaments bound to each other No relaxation Rigor mortis Pompeii, Energy & metabolism 3 sources of ATP 1. ADP phosphorylation Creatine phosphate Creatine PK Immediate & short-lived 2. Glycolysis High rate of ATP supply Low yield/mole glucose Short-lived 3. Oxidative phosphorylation Slowest Most efficient Longer lasting 20 Energy metabolism Biochemical differences in muscle fiber determine preferred energy substrate Oxidative (red muscle fibers) High blood vessel & mitochondria content ATP dependent on blood flow; O 2 & fuel Myoglobin (O 2 binding protein) Glycolytic (white muscle fibers) Low blood vessel & mitochondria content glycolytic enzymes & glycogen stores Larger in size 21 7

8 Contraction & energy Skeletal muscle fiber types Energy metabolism Major pathway of ATP generation Oxidative or glycolytic gy y Rate of force production & shortening velocity Myosin isoform rate of ATPase activity Fast or slow 22 Energy metabolism Myosin ATPase & energetics 3 Types of skeletal muscle fibers Type I Slow-twitch oxidative Low myosin-atpase activity Dense capillary network & rich myoglobin Rich mitochondrial & oxidative-enzyme content Utilize fats and carbohydrates better because of the increased reliance on oxidative metabolism High oxidative capacity & resistant to fatigue Body posture, skeletal support, endurance activities Example: soleus 23 Energy metabolism Myosin ATPase & energetics 3 Types of skeletal muscle fibers Type II Fast-twitch High myosin-atpase activity Short twitch durations & fine skilled movement Greater amounts of force production for shorter periods of time Fast oxidative (IIa) High myoglobin + mitochondria content Fast-twitch glycolytic (IIb) less mitochondria 7 >glycogen fatigue rapidly Example: gastrocnemius and vastus lateralis 24 8

9 Energy & metabolism Time to fatigue depending on fiber type 25 Energy & metabolism Fiber type determines resistance to fatigue Fatigue Muscle tension following previous contractile activity Recovery depends on duration & intensity of activity Faster after high intensity low duration (Weight lifting) Slower after low intensity long duration (distance running) Slow-oxidative fibers Resistant to fatigue Prolonged & continued (marathon) Fast-glycolytic fibers Fatigue rapidly Rapid & powerful (jump, sprint) 26 Contractile responses Relationship of AP to fiber twitch Isometric contraction Duration muscle twitch > AP Muscle; no refractory period Latent period: Tf from APto in tension Contraction time: T between initial & peak tension 27 9

10 Contractile responses Relation between muscle length & tension developed Tension developed in isometric contraction Varies with length of fiber Is maximal at resting length Total tension measured at different muscle lengths Passive tension Tension prior to contraction Active tension Total - passive tension 28 Contractile responses Length - tension relationship Maximal tension Max # of cross-bridges thick & thin filament overlap Cardiac muscle Tension developed with fiber length Smooth muscle Greater range of lengths over which maximal tension can be developed 29 Contractile responses Load-velocity-length relationship Inverse relationship between force & velocity of shortening Slow at heavy load Velocity at given load is maximal at resting length Velocity decreases if muscle is shorter or longer 30 10

11 Force of contraction by summation Spatial: # fibers recruited Small to...larger motor units Asynchronous & in tandem Temporal: frequency of stimulation Progressive frequency fused No relaxation between stimuli Tetanus 31 Cardiac muscle Structure Excitation-contraction coupling Energetics & mechanics of contraction 32 Cardiac muscle structure Cardiac muscle vs. skeletal Similarity Striations, T tubules, contractile & regulatory proteins, sliding-filament mechanism Differences # mitochondria & capillary supply ppy Intercalated disks Between fibers; cell-cell/cohesion Gap junctions (connexons) Connect cytosol of adjacent cells Extra cellular matrix Remodeling and failure Control of contraction 33 11

12 Cardiac muscle has automaticity Intrinsic ability to contract spontaneously & rhythmically Specializedsubset of cardiac muscle cells (Purkinje cells) Located in: Sino atrial (SA) & Atrioventricular (AV) nodes Bundle of His, bundle branches, & Purkinje fibers of the ventricles. 34 Cardiac muscle excitation contraction coupling Cardiac muscle functions as a syncitium Pacemaker potentials originate in SA node Depolarization via gap junctions Activation of ventricular muscle [Ca ++ ] I myocardial contraction Control of contraction Autonomic Neurotransmitters PSNS: Ach: Cholinergic (Muscarinic) receptor SNS: NE: Adrenergic (ß) receptors 35 Cardiac muscle excitation contraction coupling Cardiac muscle action potential Phase 0: open voltage-gated Na+ channels Phase 1: close Na+ channels Phase 2: slow opening of voltage-gated L-type Ca 2+ channels [DHP] Phase 3: close Ca 2+ channels & K + efflux Phase 4: resting membrane potential 36 12

13 Ca++ influx X L-type Ca++ channels Required but not sufficient for cardiac muscle contraction Ca ++ binds to SR RyR Ca ++ release [Ca ++ ]i Ca ++ Binds Troponin C Uncover actin binding sites Cross-bridge formation, cycling & contraction Relaxation x Ca ++ reuptake Phospholambdan Inhibits Ca ++ reuptake into SR + Pi removes inhibition Ca++ regulation of cardiac muscle contraction 37 2 key points for cardiac muscle contraction! Ca ++ influx X L-type Ca ++ channels required Protein phosphorylation speeds cardiac muscle relaxation 38 Hormones can modulate cardiac muscle contraction Thyroid hormone Gene transcription Ca 2+ ATPase Phospholamban Myosin Adrenergic receptors Adenylyl cyclase Na + /Ca 2+ exchanger Na + /K + ATPase Voltage gated K channels Nonnuclear actions Na +, K +, Ca 2+ ion channels 39 13

14 Cardiac muscle energetics Abundant blood supply 1 capillary/fiber Rich in mitochondria 30-40% muscle mass High myoglobin content O 2 stores Oxidative & glycolytic Fat 60% CHO 35% Ketones & AA 5% Cardiac muscle 40 Smooth muscle Structure Excitation-contraction coupling Regulation of contraction & relaxation 41 Smooth muscle structure Smooth vs. skeletal muscle Similar contractile proteins Different regulatory proteins No troponin Myosin light chain kinase Myosin light chain phosphatase Calponin & caldesmon Inhibit myosin ATPase activity Structurally Smaller cells Diagonal filament orientation Anchored to dense bodies Z lines No T-tubules & undeveloped SR Relies on extra-cellular Ca ++ Gap junctions 42 14

15 Functionally: 2 Types of SM single & multiunit Smooth muscle Single unit Visceral smooth muscle GI tract, uterus, bladder Poorly innervated Fibers linked by gap junctions Action potential propagated cell-cell Not all cells need to be stimulated Unstable membrane potential Stretch produces contraction 43 Smooth muscle Functionally: 2 Types of SM single & multinunit Multiunit Richly innervated by ANS Few or no gap junctions Little electrical coupling Fibers respond independently Capable of finer control Large airways, arteries, iris Stable membrane potential Contractile response depends # of muscle fibers activated Frequency of nerve stimulation 44 Smooth muscle Visceral SM electrical activity Unstable membrane potential Variable resting potential Slow waves can trigger bursts of action potentials Depolarization repolarization repolarization cycle Rhythmic contractions Pacemaker potentials Generated in multiple shifting foci Modulated by ANS 45 15

16 Smooth muscle action potential Variable patterns Spike Plateau Delayed repolarization prolonged Ca ++ entry & contraction Slow waves; oscillations in mv L-type Ca ++ channels active at resting mv depolarize cell enough to activate more Ca ++ channels.ca ++ influx activate K + channels.repolarization 46 Importance of Ca ++ in SM contraction *Poor SR * [Ca ++ ] I determined by: Ca ++ entering cells Voltage-gated L-type channels Store operated channels Ca ++ released by SR Ca ++, IP 3 Removal of Ca ++ Out of cell Into SR Smooth muscle 47 Smooth muscle Smooth muscle: No troponin Myosin: site of Ca ++ regulation Ca binds to Calmodulin CaCM Activates myosin light chain kinase (MLCK) *Pi + MLC myosin ATPase activity Activates CaCM-dependent kinase *Pi + calponin calponin inhibition of myosin ATPase 48 16

17 49 Multiple mechanisms regulate SM excitation-contraction coupling Electrical depolarization Voltage-gated Ca ++ channels L-type Ca ++ channels Chemical stimuli Hormones, NT, local factors Receptor-mediated Mechanical Passive stretching Myogenic response 50 Smooth muscle Neurotransmitters & hormones Do not have specialized end plate region Varicosities filled with neurotransmitters NT released when AP passes the varicosity Varicosities from one axon may be in more than one fiber Single fiber may be located near SNS & PSNS varicosities 51 17

18 Receptor type determines response Smooth muscle Neural stimulation of smooth muscle Norepinephrine contracts vascular smooth muscle (α 1 receptors skin) Acetylcholine contracts intestinal smooth muscle Phospholipase C & IP 3 intracellular Ca ++ Ca CM MLCK Epinephrine relaxes bronchial, uterine & vascular smooth muscle (skeletal muscle) camp PKA inhibits activity of MLCK 53 Smooth muscle Hormones modulate SM function & structure High estrogen: SM hypertrophy & gap junctions Oxytocin stimulates uterine contractility 54 18

19 How is blood pressure controlled? AVP Ang II Norepi AT AVPR 1R α1 AR VGCC 55 Key features of smooth muscle Slow cross bridge formation and cycling rate Maintains tension for prolonged periods Latch or tonic state Minimal ATP needs (oxidative & gy glycolytic) y Increased tension developed over greater range of length Contracts in response to stretch Does not need action potential to contract 56 Review Table Characteristic Skeletal Cardiac Smooth Thick & thin filaments Yes Yes Yes Striated pattern Yes Yes No T-tubules Yes Yes No Sarcoplasmic as c Reticulum u Gap junctions no Yes Yes Ca ++ source SR EC & SR EC & SR Site of Ca ++ regulation Troponin Troponin Myosin Hormone effects no yes yes Stretch induces contraction no no yes 57 19